Since you will be practicing the art and science of "water
gardening", it is a wise idea to know what your local water
contains. Contact your local water company and ask for their
water quality analysis. If your water comes from a well, you will
most likely have to send it out to a lab for analysis on your own.
The most important factor affecting water quality is its relative
"hardness" or "softness". Hand water means that there is a lot of
dissolved mineral content, primarily calcium carbonate, which is
often seen as scale on hot water pipes. Soft water is generally very
pure or low in dissolved solids. Distilled (or deionized ) water, or
water that has been through a reverse osmosis filter, are all
considered soft. Most commercially available hydroponic nutrients
are made for soft water. However, if you have hard water, there
are some nutrient products made for hard water as well.